This project has two major goals: 1) to study tissue-specific initiation factors which are involved in posttranscriptional regulation of protein synthesis of immunoglobulins by cell cultured mouse myeloma cells, and 2) to study the inactive cytoplasmic form of immunoglobulin messenger RNA. The first aim will be pursued by using synchronized myeloma cells which show a burst of immunoglobulin synthesis during late G-1 phase of the cell cycle. Initiation factors isolated during this stage of the cell cycle have been shown to stimulate immunoglobulin synthesis in a heterologous system to a much greater extent than initiation factors isolated during G-2 phase. Total m-RNA coding for immunoglobulins is more or less constant during various phases of the cell cycle in synchronized myeloma cells. However, the m-RNA coding for immunoglobulins associated with actively synthesizing membrane-bound polysomes is greatest during late G-1 phase. We intend to study the inactive form during other phases of the cell cycle.